1. Technical Field
This invention relates generally to protection circuits for rechargeable battery packs, and more specifically to protection circuits that enhance the reliability and performance of a battery pack in volatile environmental conditions.
2. Background Art
Portable electronic devices, like cellular telephones, pagers and two-way radios for example, derive their portability from rechargeable batteries. Such batteries allow these devices to slip the surly bonds of wall mounted power supplies and wirelessly touch the hand of the user wherever he may be.
While many people may think that a rechargeable battery is simply a cell and a plastic housing, nothing could be further from the truth. Rechargeable battery packs often include circuit boards, electronic circuitry, mechanical assemblies and electromechanical protection components. The circuits employed in rechargeable battery packs include charging circuits that control current, fuel gauging circuits, temperature measurement circuits and indicator circuits, just to name a few. Simply put, a battery pack is a complex system of components working in harmony to safely deliver power to a portable electronic device.
One of the most fundamental circuits in a battery pack is the protection circuit. Rechargeable battery performance, especially with respect to those having cells constructed of lithium-based materials, may be severely compromised if the cell within the battery pack is over or under charged. For this reason, most all battery packs today include one form of lithium ion protection IC or another.
Typical lithium ion protection ICs include voltage and current limits. As such, when the voltage across the cell in a battery pack becomes too high or too low, the lithium ion protection IC will open switches within the pack, thereby “turning off” the battery pack. Similarly, if the current flowing either into or out of the cell gets too high, the lithium ion protection IC will turn off the battery pack.
Despite these voltage and current safety mechanisms, new concerns are arising from “thermal” situations, especially in volatile environments. Standards boards in various countries are enacting strict standards to ensure that battery packs remain reliably operable where, for example, combustible gasses may be present in the atmosphere. One such standard is the Atmospheric Explosive (ATEX) standard set forth by the European Union. This stringent standard sets forth current, voltage and charging limits for rechargeable batteries that today's lithium ion protection ICs simply can not meet. For example, while a conventional lithium ion protection IC will disable charging when cells become over charged, there is an inherent propagation delay associated with this disabling action. Under certain short circuit conditions, conventional lithium ion protection ICs simply do not open fast enough to meet the ATEX requirements.
There is thus a need for an improved battery pack having a robust protection system capable of meeting the operational demands of battery packs in volatile environments.